buffer responses to compute programs in a TQueue

This avoids a potential problem where the program sends several INPUT
before reading responses, so flushing the respose to the pipe could
block. It's unlikely, but seemed worth making sure it can't happen.

Remote/Compute.hs

@@ -435,10 +435,12 @@ runComputeProgram (ComputeProgram program) state (ImmutableState immutablestate)
 		showOutput
 		starttime <- liftIO currentMonotonicTimestamp
 		let startresult = ComputeProgramResult state False False False
-		result <- withmeterfile $ \meterfile -> bracket
-			(liftIO $ createProcess pr)
-			(liftIO . cleanupProcess)
-			(getinput tmpdir subdir startresult meterfile)
+		result <- withmeterfile $ \meterfile -> 
+			bracket
+				(liftIO $ createProcess pr)
+				(liftIO . cleanupProcess) $ \p -> 
+					withoutputv p $
+						getinput tmpdir subdir startresult meterfile p
 		endtime <- liftIO currentMonotonicTimestamp
 		liftIO $ checkoutputs result subdir
 		cont result subdir (calcduration starttime endtime)
@@ -453,14 +455,14 @@ runComputeProgram (ComputeProgram program) state (ImmutableState immutablestate)
 			, return tmpdir
 			)
 	
-	getinput tmpdir subdir result meterfile p = 
+	getinput tmpdir subdir result meterfile p outputv = 
 		liftIO (hGetLineUntilExitOrEOF (processHandle p) (stdoutHandle p)) >>= \case
 			Just l
-				| null l -> getinput tmpdir subdir result meterfile p
+				| null l -> getinput tmpdir subdir result meterfile p outputv
 				| otherwise -> do
 					fastDebug "Compute" ("< " ++ l)
-					result' <- parseoutput p tmpdir subdir result meterfile l
-					getinput tmpdir subdir result' meterfile p
+					result' <- parseoutput outputv tmpdir subdir result meterfile l
+					getinput tmpdir subdir result' meterfile p outputv
 			Nothing -> do
 				liftIO $ hClose (stdoutHandle p)
 				liftIO $ hClose (stdinHandle p)
@@ -468,19 +470,14 @@ runComputeProgram (ComputeProgram program) state (ImmutableState immutablestate)
 					giveup $ program ++ " exited unsuccessfully"
 				return result
 	
-	sendresponse p s = do
-		fastDebug "Compute" ("> " ++ s)
-		liftIO $ hPutStrLn (stdinHandle p) s
-		liftIO $ hFlush (stdinHandle p)
-
-	parseoutput p tmpdir subdir result meterfile l = case Proto.parseMessage l of
-		Just (ProcessInput f) -> handleinput f False p tmpdir subdir result
-		Just (ProcessInputRequired f) -> handleinput f True p tmpdir subdir result
+	parseoutput outputv tmpdir subdir result meterfile l = case Proto.parseMessage l of
+		Just (ProcessInput f) -> handleinput f False outputv tmpdir subdir result
+		Just (ProcessInputRequired f) -> handleinput f True outputv tmpdir subdir result
 		Just (ProcessOutput f) -> do
 			let f' = toOsPath f
 			checksafefile tmpdir subdir f' "output"
 			-- Modify filename so eg "-foo" becomes "./-foo"
-			sendresponse p $ toCommand' (File f)
+			sendresponse outputv $ toCommand' (File f)
 			-- If the output file is in a subdirectory, make
 			-- the directories so the compute program doesn't
 			-- need to.
@@ -508,7 +505,7 @@ runComputeProgram (ComputeProgram program) state (ImmutableState immutablestate)
 		Just ProcessSandbox -> do
 			sandboxpath <- liftIO $ fromOsPath <$>
 				relPathDirToFile subdir tmpdir
-			sendresponse p $
+			sendresponse outputv $
 				if null sandboxpath
 					then "."
 					else sandboxpath
@@ -516,7 +513,7 @@ runComputeProgram (ComputeProgram program) state (ImmutableState immutablestate)
 		Nothing -> giveup $
 			program ++ " output an unparseable line: \"" ++ l ++ "\""
 
-	handleinput f required p tmpdir subdir result = do
+	handleinput f required outputv tmpdir subdir result = do
 		let f' = toOsPath f
 		let knowninput = M.member f'
 			(computeInputs (computeState result))
@@ -534,7 +531,7 @@ runComputeProgram (ComputeProgram program) state (ImmutableState immutablestate)
 						mkrel $ pure obj
 				Just (Left gitsha) -> 
 					mkrel $ populategitsha gitsha tmpdir
-			sendresponse p $
+			sendresponse outputv $
 				maybe "" fromOsPath mp
 			let result' = result
 				{ computeInputsUnavailable = 
@@ -630,6 +627,28 @@ runComputeProgram (ComputeProgram program) state (ImmutableState immutablestate)
 			Just sz ->
 				progress $ BytesProcessed $ floor $ 
 					fromIntegral sz * percent / 100
+	
+	withoutputv p a = do
+		outputv <- liftIO $ atomically newTQueue
+		let cleanup pid = do
+			atomically $ writeTQueue outputv Nothing
+			wait pid
+		bracket 
+			(liftIO $ async $ sendoutput' p outputv)
+			(liftIO . cleanup)
+			(const $ a outputv)
+
+	sendoutput' p outputv =
+		atomically (readTQueue outputv) >>= \case
+			Nothing -> return ()
+			Just s -> do
+				liftIO $ hPutStrLn (stdinHandle p) s
+				liftIO $ hFlush (stdinHandle p)
+				sendoutput' p outputv
+
+	sendresponse outputv s = do
+		fastDebug "Compute" ("> " ++ s)
+		liftIO $ atomically $ writeTQueue outputv (Just s)
 
 computationBehaviorChangeError :: ComputeProgram -> String -> OsPath -> Annex a
 computationBehaviorChangeError (ComputeProgram program) requestdesc p =

doc/todo/compute_special_remote_remaining_todos.mdwn

@@ -1,11 +1,6 @@
 This is the remainder of my todo list while I was building the
 compute special remote. --[[Joey]]
 
-* git-annex responds to each INPUT immediately, and flushes stdout.
-  This could cause problems if the program is sending several INPUT
-  first, before reading responses, as is documented it should do to allow
-  for parallel get of the input files.
-
 * write a tip showing how to use this
 
 * Support parallel get of input files. The design allows for this,